High performance purified natural zeolite pigment for papermaking and paper coating

ABSTRACT

A high performance purified natural zeolite pigment composition for use in papermaking and paper coating is disclosed. Use of the pigment facilitates manufacture of coated ink jet and digital printing papers with improved quality and economics. The novel zeolite pigment composition can also be used as a supplementary pigment to improve the properties of coated paper and paperboard for flexographic and water-based gravure printing. When used as filler, the novel zeolite pigment composition is readily retained and eliminates print-through in uncoated papers. The novel zeolite pigment is low in abrasion and provides improved coefficient of friction. The novel zeolite pigment is also useful as a microparticulate retention aid in papermaking and as an additive to improve the performance of deinking processes.

This is a divisional patent application claiming priority to U.S. patentapplication Ser. No. 09/922,147 filed Aug. 3, 2001 now U.S. Pat. No.6,616,748.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Applicant's invention relates to a purified natural zeolite pigmentcomposition for filling and/or coating paper. More particularly, thepresent invention relates to a purified natural zeolite pigmentcomposition that can be used for coating paper that produces a paperthat exhibits improved characteristics over existing uncoated and coatedpapers made with other pigments.

2. Background Information

Pigments are used in papermaking and paper coating to improve theappearance, optical properties and printability of papers. Commonly usedpigments include kaolin clay, calcium carbonate, titanium dioxide,alumina trihydrate and polystyrene. These pigments are useful inmanufacture of conventional printing and writing papers and paperboardsthat are printed or imaged by common processes including offsetlithography, gravure and xerography. Recently developed imagingtechnology has created needs for new types of coated and uncoated paperswith properties not achievable with conventional pigments. Ink jetprinting is a useful example.

Ink jet printing technology has undergone several changes in addressingthe demands of existing and future digital printing applications thatrequire high quality printed images. High quality ink jet printingtypically occurs on coated paper; therefore, to produce such highquality printed images the coating composition and the ink formulationmust be considered.

Current ink jet papers rely on the novel properties of the coatingmaterial to create desired properties to dry and set the ink solutions.Jet inks typically contain 2.5% by weight of organic dyes. The dye isfixed to the paper surface either by evaporation of a base such asammonia, by migration of a base such as diethanolamine into the paper,or by changes in ionic environment when the ink meets the coatingmaterial layer.

The paper must exhibit unique properties in order to produce a highquality printed image when the ink is fixed to the paper surface. Oncethe ink drop is accepted by the paper, the ink must adhere to the paperand spread minimally in all directions to generate sharp edges for printcontrast¹ and image fidelity. The paper must be smooth to give highprint densities². In addition, the paper should minimize bleeding³ andwicking while promoting the absorption of ink to set the dye onto thecoated surface since this promotes higher print densities. Ink jetdroplets must be adsorbed quickly to avoid image smearing and multipledrop splatter. The dyes should be deposited near the paper surface tomaximize color density and contrast while minimizing show through⁴.

¹ Contrast is defined as the tonal change in color from light to dark.

² Density is defined as the degree of color or darkness of an image.

³ Bleeding is defined as ink traveling into the sheet.

⁴ Show through is defined as printing that is visible from the backsideof a sheet, or the next sheet, under normal lighting conditions.

Coating, which generally contains pigment, binders, and additives, isapplied to the paper surface to improve the properties of the paper. Theink interacts with the coating to produce a high quality image. Thecoating prevents the ink from penetrating into the substrate. Morespecifically, the coating can optimize drying time for high watercontent dyes and separate the water-soluble organic dyes from the watervehicle and hold the dye on the surface so it doesn't strike through tothe base sheet. Smoothness and thickness of the coating layer are twoimportant physical properties that impact print quality. Pore structureand contact angle wettability effect print quality by preventing inkspreading. In order to prevent wicking and feathering⁵, it is importantthat the thickness of the coating layer be homogenous to a scale of afew microns in depth which also helps in the absorption of successivedroplets of ink at high delivery rates and any water present.

⁵ Feathering is defined as the spreading of ink at the edges of printedtype, caused by irregularities in the ink or its distribution.

Paper made for ink jet printing should have a hydrophilic, high porositysurface with no macroscopic structure in order to absorb ink jetdroplets quickly with little spreading, wicking or dye penetration.Therefore the preferred coating for the paper surface should contain ahighly porous, high surface area pigment that wets almost instantly withwater. If the coating has sufficient thickness and void volume, itshould be able to absorb successive droplets in multicolor printing atthe highest delivery rates of commercial ink jet printing. The dyeshould react with the coating material to make it waterfast and rubresistant. The coating should have near neutral or alkaline pH to avoidshifts from the intended color of the dyes.

The rate of ink penetration has a large effect on final optical densitythrough its effect on drying time and setting of the dye on the coatedsurface. The rate of ink penetration can be explained by the LucasWashburn Equation of capillary flow:

I ² =yr(cosθ)t/4v

where I is the depth of ink penetration, r is the pore radius, t istime, y is the surface tension, θ is the contact angle, and v is theviscosity of the ink. In generating high print quality, the rate of inkpenetration must be modified to allow sufficient wetting to occur. Thehydrophilic/hydrophobic surface chemistry of the coating plays animportant role in the development of image quality through the controlof dot gain. Sufficient dot gain requires the dot spreading on a smoothsurface and is a function of contact angle. The contact angle is itselfa function of the interactions between the surface tension of theliquid, surface vapor, and liquid vapor interfaces. The determination ofsufficient dot gain can be characterized through the surface tension ofthe interfaces from Young's equation:

Y _(slv) =Y _(si+) Y _(iv)cosθ

This equation evaluates the development of the contact angle whichcontrols spread of liquid through the surface tensions involved. If thecontact angle is less than 90 degrees, surface roughness will reduce thecontact angle even more. Whereas if the contact angle is greater than 90degrees the surface roughness will increase the contact angle. Porosityalso effects the measured contact angle.

The interactions between ink and the coated substrate play a vital rolein producing images that are long lasting, well defined and of highstrength regardless of printer application. The main interaction occursat the surface of the substrate, where the type of bonding that occursbetween the colorant and the media dictates the final print quality. Theinteractions that take place between the colorant and the plain paperare controlled by hydrogen bonding and Van der Waals forces, while ionicand electrostatic forces are responsible for the interactions betweenthe colorant and the coated paper.

Hydrogen bonding is the most significant bonding that takes placebetween color and media, where cellulosic material is involved. For alarge dye molecule, a large number of sites are available for hydrogenbonding which encourage the interaction between the colorant and themedia. Hydrogen bonding between color and media increases the strengthof the color binding on the media. Furthermore, the hydroxyl groups ofthe cellulose may interact with the δ cloud of an aromatic group on thecolorant by hydrogen bonding.

Van der Waals forces are very weak when the interacting groups are farapart and a weak repulsion typically exists between the media andanionic dyes. The interaction between colorant and media becomes strongas the dyes start penetrating into the base sheet.

Electrostatic forces occur due to coloumbic attraction. The cationicgroups on the media, such as Ti³⁺, Al³⁺, and Ca²⁺attract anionic dyes,such as water-soluble groups of SO₃ ²⁻, COO , and PO₄ ³⁻. The result isstrong attraction between these groups, which causes an effectiveimmobilization of the dye molecules, resulting in excellent printquality.

The δ—δ interactions are very strong interactions that typically occurbetween dye molecules. These interactions normally generate either dyeaggregation or crystallization⁶. If dye-dye interactions on the papersubstrate are stronger than dye-paper interactions, dye may aggregate onthe substrate causing printing problems. Thus a strong interactionbetween colorant and media is required.

⁶ Crystallization is a condition in which a dried ink film repels asecond ink which must be printed on top of it.

Hydrogen bonding and Van der Waals forces are the main interactions thatoccur in plain papers. Plain papers mainly consist of cellulose andtherefore the main interactions are between the color and the cellulose.The penetration of color into the substrate will be controlled bycapillary adsorption. If the paper has been internally or surface sizedthe rate of penetration of the colorant will be decreased which may leadto some ink bleeding and feathering problems.

The interaction of the colorant with coated paper is different however.The selection of the coating and ink formulation will have a significanteffect on the ink absorption rate, image quality, and water/lightfastness properties of the liquid ink. Electrostatic or ionicinteractions play the key role in colorant coated paper interactions.Electrostatic interaction is stronger than hydrogen bonding and Van derWaals interactions. These interactions are more efficient, as thecolorant is fixed in the vicinity where it was printed. The nature ofthe anionic dyes and the oxides will determine the print quality of inkjet printing since electrostatic interactions of the colorant withcoated media occur between the anionic groups of the dyes and oxides.The binding energies of the dyes are greatly increased by electrostaticinteractions resulting in high bonding strength.

Existing coated ink jet papers are mainly dependent on amorphous andgelled silica, which possess high micro porosity and macro porosity. Theporous coating structure provides the driving force for the rapiddiffusion of ink liquid into the coating layer and internal pore volumeof the coating for storing large amounts of ink. These two propertiesinteract to set the anionic dye at or near the coating surface,generating higher optical printing densities. The high surface area ofthe silica requires a strong binder to maintain adhesion to the paperand cohesion within the coating structure. Therefore, polyvinyl alcohol,the strongest binder available, is used.

Unfortunately, the current use of silica and polyvinyl alcohol hasseveral limitations that effect the coating. The internal porosity ofthe silica pigments and the degree of hydrolysis of the polyvinylalcohol limits running the coating solids at 20%. Silica pigments poseproduction problems and high cost because they must be coated atrelatively slow speeds. Coating solids level is a major limiting factorwith silica pigments because of viscosity, water absorption, and dryingissues. Silica slurries alone do not usually flow well at levels above15 to 20% solids, so dispersants are used to increase theirconcentrations. Also, silica has a great affinity for water given itshigh pore volume so it forms a paste as water is added until all thevoids are filled. Only then is it fluid enough for the coatingformulation. This behavior decreases the vehicle available for theslurry, so formulators must start at a lower solids concentration. Theabsorbed water in the pores also demands extra energy during drying.Calcium carbonate is another material sparingly used for ink jet printercoatings that dry similar to silica, but its surface area and voidvolume are much lower than silica—resulting in inferior image quality.It is also abrasive and can exhibit poor coater runnability. Its use islimited to cast coated ink jet papers for glossy photo prints where itis used as a supplementary pigment to silica.

With the compositions for coating paper currently on the market higherquality coated ink jet papers must be coated off-machine and are notcost effective. Producing a paper sheet with the desired properties isdifficult due to the need to find ways to coat ink jet paper on-machineat commercial speeds with no loss in quality. The preferred finished inkjet paper should be smooth, strong, opaque, bright, and able to handlethe demands of ink jet printing while providing excellent print results,such as excellent ink adherence, high scratch and ink resistance, andbleed control for sharp edges. It was therefore necessary to develop thecomposition for coating paper of the present invention that produces acoated paper that overcomes the disadvantages of the existing art whilepresenting a high print quality image at a reduced cost. Morespecifically, the present invention contemplates substituting a zeolitepigment for silica in matte ink jet coating formulations.

A zeolite pigment that possesses the desirable combination ofbrightness, color, particle size distribution, surface area, internalvoid volume, rheology and hardness could also be useful in overcomingthe limitations of conventional and other specialty pigments in variouspapermaking and paper coating applications including but not limited to:(1) toner bond improvement in laser and other dry toner imaged digitalpapers; (2) elimination of smudging and improvement of print quality indirect print flexography on coated linerboard used in corrugatedcontainers; (3) elimination of print through on newsprint and ultralight weight coated papers; (4) improvement of dot fidelity and printquality on coated rotogravure printing papers; (5) low abrasion extenderfor titanium dioxide pigments; (6) improvement of coefficient offriction of paper and paperboard; (7) production of technical specialtypapers such as anti-tarnish, gas filtration, and absorbent papers withimproved properties and lower cost of manufacture; (8) more economicalmicroparticulate retention system chemistry; (9) additive to improve theefficiency of deinking systems.

Zeolites are crystalline, hydrated aluminosilicates of the alkali andalkaline earth metals. More particularly, zeolites are frameworksilicates consisting of interlocking tetrahedrons of SiO₄ and AlO₄. Inorder to constitute a zeolite the ratio of silicon and aluminum tooxygen must be ½. The alumino-silicates structure is negatively chargedand attracts the positive cations that reside within. When exposed tohigher charged ions of a new element, zeolites will exchange the lowercharged element contained within the zeolite for a higher chargedelement. Unlike most other tectosilicates, zeolites have large vacantspaces or cages in their structures that allow space for large cationssuch as sodium, potassium, barium, and calcium and relatively largemolecules and cationic molecules, such as water, ammonia, carbonateions, and nitrate ions. In most useful zeolites, the spaces areinterconnected and form long wide channels of varying sizes depending onthe mineral. These channels allow ease of movement of the resident ionsand molecules into and out of the structure.

Zeolites are characterized by 1) a high degree of hydration, 2) lowdensity and large void volume when dehydrated, 3) stability of thecrystal structure of many zeolites when dehydrated, 4) uniform molecularsized channels in the dehydrated crystals, 5) ability to absorb gasesand vapors, 6) catalytic properties, and 7) cation exchange properties.

The use of natural zeolites in paper making has a long history, but hasbeen almost unique to Japan where zeolite has been used as filler toimprove bulkiness and printability. Natural zeolites have also been usedas fillers for paper in Hungary. These natural zeolites however are alow brightness material and this renders it unsatisfactory forapplication in the United States on coated ink jet paper where highbrightness is expected.

Numerous families of natural zeolites exist and each has varyingcharacteristics. Unfortunately, natural zeolites exhibit nonuniformproperties that makes them difficult to work with in many applicationsbecause ores from one location can vary with any other. It is howeverpossible to manufacture zeolites with uniform properties. The preferredzeolite for use in the present invention is a processed form of thenatural mineral clinoptilolite which is a hydrated sodium potassiumcalcium aluminum silicate having the formula (Na, K, Ca)₂₋₃ Al₃ (Al,Si)₂Si₁₃)₃₆-12H₂O. This zeolite is within the family Heulandite that alsoincludes the mineral heulandite which is a hydrated sodium calciumaluminum silicate. The physical characteristics of raw clinoptiloliteare listed in Table 1.

TABLE 1 PHYSICAL CHARACTERISTICS OF CLINOPTILOLITE Color is colorless,white, pink, yellow, reddish and pale brown. Luster is vitreous topearly on the most prominent pinacoid face and on cleavage surfaces.Transparency: Crystals are transparent to translucent. Crystal System ismonoclinic; 2/m. Crystal Habits include blocky or tabular crystals withgood monoclinic crystal form. More tabular and proportioned thanheulandite. Also commonly found in acicular (needle thin) crystalsprays. Cleavage is perfect in one direction parallel to the prominentpinacoid face. Fracture is uneven. Hardness is 3.5-4, maybe softer oncleavage surfaces. Specific Gravity is approximately 2.2 Streak iswhite.

Clinoptilolite's structure is sheet like with a tectosilicate structurewhere every oxygen is connected to either a silicon or an aluminum ion(at a ratio of [Al+Si]/0=½). The sheets are connected to each other by afew bonds that are relatively widely separated from each other. Thesheets contain open rings of alternating eight and ten sides. Theserings stack together from sheet to sheet to form channels throughout thecrystal structure. The size of these channels controls the size of themolecules or ions that can pass through them. Clinoptilolite is wellsuited for various applications, such as in paper coating compositions,because it exhibits large pore space, high resistance to extremetemperatures, and has a chemically neutral structure.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel purifiednatural zeolite pigment for coated ink jet papers and digital printingpapers to replace silica pigments.

Another object of the present invention is to provide a novel purifiednatural zeolite pigment that can be used as a specialty coating pigmentin coated linerboard for direct post print flexography to preventsmudging and to improve image fidelity.

Still another object of the present invention is to provide a novelpurified natural zeolite pigment that can act as a supplementary coatingpigment in ultra lightweight coated publication papers.

It is yet another object of the present invention to provide a novelpurified natural zeolite pigment that can act as a supplementary coatingpigment for water based gravure⁷ printing papers.

⁷ Gravure printing is a method of printing using etched metal cylinders.

An additional object of the present invention is to provide a novelpurified natural zeolite pigment that can replace calcined kaolin as atitanium dioxide extender in coated recycled paperboard and coated solidunbleached sulfate (SUS) beverage carrier stock.

It is still another object of the present invention to provide a novelpurified natural zeolite pigment that can act as filler in newsprint toprevent print-through.

It is yet another object of the present invention to provide a novelpurified natural zeolite pigment that can act as filler in specialtytechnical papers such as anti-tarnish, gas filtration, filter, andabsorbent papers.

Another object of the present invention is to provide a novel purifiednatural zeolite pigment that can be used as a microparticulate retentionaid.

Still another object of the present invention is to provide a novelpurified natural zeolite pigment that can be used as a deinking aid incombination flotation-washing systems.

Yet another object of the present invention is to provide a novelpurified natural zeolite pigment that can be used as a coefficient offriction (COF) control aid in recycled linerboard.

Another object of the present invention is to provide a novel purifiednatural zeolite pigment for use in a coating composition that hasimproved rheology compared to silica and other specialty pigments.

Still another object of the present invention is to provide a novelpurified natural zeolite pigment for use in a coating composition thatimproves coater runnability.

It is yet another object of the present invention to provide a novelpurified natural zeolite pigment for use in a coating composition thathas decreased energy consumption in drying.

It is an object of the present invention to provide a novel compositionfor coating paper that has water slurries with a higher percentage ofsolids and good shear thinning rheology compared to existingcompositions.

Another object of the present invention is to provide a novelcomposition for coating paper that has higher coating formulation solidscompared to existing compositions.

Still another object of the present invention is to provide a novelcomposition for coating paper that has enhanced on-machine coating runability and therefore enhanced production rates over existingcompositions.

It is yet another object of the present invention to provide a novelcomposition for coating paper that has low Einlehner abrasion whichresults in reduced wear to process equipment and no metallic marks areleft on the paper by the gripper bars.

Another object of the present invention is to provide a novelcomposition for coating paper that has a low bulk density.

Still another object of the present invention is to provide a novelcomposition for coating paper that has faster on-machine drying ratesbecause of higher percent solid coatings than existing compositionswhich results in lower drying costs and reduced print smear.

Yet another object of the present invention is to provide a novelcomposition for coating paper that has a low crystalline silica content.

It is another object of the present invention to provide a novelcomposition for coating paper that coats with essentially no dusting.

It is still another object of the present invention to provide a novelcomposition for coating paper that has improved first pass retention inpaper machine trials compared to existing compositions.

Another object of the present invention is to provide a novelcomposition for coating paper that has improved optical/reflectivedensities of four-color cyan, magenta, yellow, black (CMYK) ink jetprint.

An additional object of the present invention is to provide a novelcomposition for coating paper that makes lighter coat weights possiblebecause of higher internal void volume.

Still another object of the present invention is to provide a novelcomposition for coating paper with a slightly basic pH.

Yet another object of the present invention is to provide a novelcomposition for coating paper that has a high brightness of 90% or more.

Another object of the present invention is to provide a novelcomposition for coating paper that has a narrow particle sizedistribution with few fines.

An additional object of the present invention is to provide a novelcomposition for coating paper that improves ink jet print density.

It is yet another object of the present invention to provide a novelcomposition for coating paper that improves ink receptivity in printingpapers.

Still another object of the present invention is to provide a novelcomposition for coating paper that has improved opacity.

An additional object of the present invention is to provide a novelcomposition for coating paper that has less soak-in and reducedroughening of the base sheet during application which results in asmoother coated sheet.

Another object of the present invention is to provide a novelcomposition for coating paper that allows higher operating speeds andhigher production rates.

It is still an additional object of the present invention to provide anovel composition for coating paper that has the capability to coat onhigh speed paper machines rather than only on low speed off machinecoating lines which reduces waste and costs.

In satisfaction of these and related objectives, Applicant's presentinvention provides a purified natural zeolite pigment composition forcoating and/or filling of paper. Applicant's invention permits itspractitioner to manufacture coated paper for use in ink jet printersthat exhibits improved characteristics over existing uncoated and coatedpapers such as high print quality images and reduced cost. It alsopermits the practitioner to make other specialty and technical papersthat exhibit quality and economic advantages over papers made withexisting technology and commercially available materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the dynamic contact angle versus time in secondsfor coating compositions both with and without the zeolite pigment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The processed zeolite used in the present invention has several specificcharacteristics as indicated in Table 2.

TABLE 2 Characteristics of Zeolite Pigment Samples Zeolite PigmentZeolite Pigment Specification Sample 1 Sample 2 GE Brightness⁸ % 94+ 90+L⁹ 98.46 98.00 a 0.43 0.44 b 1.25 1.72 Yellowness Index 2.48 2.05Particle Size μ, <D90 2.0 2.0 Einlehner Abrasion, mg loss 12 18 LooseDensity, lbs./cu.ft. 8 8 Packed Density, lbs./cu.ft. 12 12 RefractiveIndex 1.48 1.48 Surface Area, sq.m./g. 40-50 40-50 Oil Absorption,lbs./100 lbs. 70-80 70-80 Density, g/cc 2.2 2.2 pH in Water 5.0 8.5Cation Exchange Capacity 1.6-1.8 1.8-2.0 Brookfield Viscosity, 20 rpm1000 cPs 1000 cPs @ 40% solids* Hercules Viscosity 1 dyne 1 dyne ⁸GEBrightness is a directional brightness measurement utilizing essentiallyparallel beams of light with a wavelength of 457 nm to illuminate thepaper surface at an angle of 45°. It is also referred to as TAPPIBrightness. GE or TAPPI Brightness is the value obtained by TAPPI Testmethod T646 om-94 “Brightness of Clay and Other Mineral Pigments” (45degrees/0 degrees). ⁹L, a, b values are the chromacity coordinates orcolor values of paper or paperboard measures with tristimulus filtercolorimeters or spectrophotometers incorporating direction (45°/0°)geometry and CIE (International Commission on Illumination) illuminantC. “L” represents lightness, increasing from zero for black to 100 forwhite; “a” represents redness when plus, greenness when minus and zerofor gray; “b” represents yellowness when plus, # blueness when minus,and zero for gray. This is referred to as TAPPI Test Method T 524 om-94“Color of Paper and Paperboard (45°/0° Geometry).” @ 1100 rpm**Nonoptimized dispersion in water

In evaluating the usefulness of the present zeolite, its materialproperties were tested. The first step was to determine whether thezeolite pigment could be dispersed using commonly available dispersants.

The colorants used in aqueous ink jet printer inks are anionic. Acationic material is used along with the pigment to fix the printedimage to the paper. It is most desirable that an ink jet pigment bedispersible with a cationic dispersant with the dispersant providingdual functionality in the coating. The standard cationic dispersant forsilica ink jet coatings is poly-dimethyl-diallyl ammonium chloride(DMDAAC) which has a common usage rate of 5% on dry pigment.

Evaluation of dispersants was done by adding a pre-weighed amount ofpigment (enough to yield a 50% solids dispersion) to water under highshear using a Cowles Dissolver™ disperser.¹⁰ The pigment was addedslowly to the water until the viscosity of the pigment began tosubstantially increase. This occurred around the 46% solids point. Thedispersant being evaluated was then added to drop the viscosity, and theremainder of the pigment was added. Samples of the pigment dispersionwere taken for Brookfield viscosity and Hercules high-shear rheologytesting. The final solids content was determined by oven drying a sampleof the pigment dispersion.

¹⁰ Cowles Dissolver™ is the trade name for an open rotor high shearmixer-disperser manufactured by Morehouse-Cowles Corp., Fullerton,Calif.

The present zeolite was successfully dispersed with 5% DMDAAC to provide50.7% slurry solids with Brookfield viscosity of 414 cPs at 100 rpm witha No.4 spindle. The 20-rpm viscosity at 50.7% solids was 1520 cPs. Thelower viscosity at 100 rpm indicates that the present zeolite pigmenthas a shearthinning rheology which is highly desirable for applicationon blade, rod, and metering size press coaters. With silica pigments,such as Grace-Davison's Sylojet™, J. M. Huber's Optisil™ or ICICrosfield's Gasil®, use of 5% DMDAAC provides dispersion at<30% maximumsolids. The lower solids of silica dispersions severely limitapplication solids for formulated coating colors. Silica pigments arealso known to be dilatant or shear thickening, which causes runningproblems on blade and rod coaters. The rheology of silica pigments makesit impractical to run them in on-machine metering size press coatings atsolids content high enough to prevent soak-in and binder migration.Hercules high-shear rheograms of the present zeolite pigment confirmedthat the present pigment provides rheology suitable for good coaterrunnability and sheet surface quality.

For use in applications other than ink jet printing—for example as anadjunct pigment in coating formulations including kaolin clay, calciumcarbonate and titanium dioxide—it is desirable that the zeolite pigmentbe dispersible with a standard dispersant used for conventionalpigments. The present zeolite was also successfully dispersed with 2%AMP-95™ (2-amino, 2-methyl, 1-propanol)¹¹ to provide a stable dispersionat 50.34% solids with Brookfield viscosity of 470 cPs at 100 rpm with aNo.

¹¹ AMP-95™ (2-amino, 2-methyl, 1-propanol), a commercial dispersantmanufactured by Angus Chemical Company, Buffalo Grove, Ill. 4 spindle.The 20-rpm viscosity at 50.34% solids was 1680 cPs. Hercules high-shearrheograms showed the AMP-95™ zeolite dispersion to be thixotropic andshear thinning—desirable rheology for paper and paperboard coating.

Drawdowns¹² of pure dispersed zeolite pigment of the present inventionon a 76.6% brightness base sheet gave 86.0% GE brightness.

¹² A drawdown is a coating chemist's method of coating a small sheet ofpaper for testing purposes. A bead of pigment slurry or coating isplaced on the paper and drawn down with a wire wound rod which providesuniform volumetric metering onto the sheet surface. Coat weight per unitarea is controlled by the choice of wire winding i.e. larger or smallgrooves for the coating to flow between adjacent wire grooves.

Drawdowns were made with various ratios of pigment to PVOH (polyvinylalcohol) binder to determine the CPVC.¹³ The CPVC was found to be 50%,that is a pigment to binder ratio of 1:1.

¹³ CPVC (Critical Pigment Volume Concentration) is the pigment volumeconcentration at which the binder just fills the volume between pigmentparticles.

Dusting was also evaluated. Dusting¹⁴ (sometimes called “rub off” or“chalking” is a major potential problem with ink jet papers made withsilica pigments. Many coaters find that they must add polyvinylpyrolidone (PVP) to control dusting. Drawdowns were made with thepresent zeolite and polyvinyl alcohol at pigment binder ratios up to14:1 to evaluate the dusting potential. No PVP was added. The presentzeolite pigment coatings did not dust at pigment to binder ratios up to14:1, which provides a significant performance advantage.

¹⁴ Dusting occurs when the coating pigment particles are not adequatelybound to one another and/or the sheet surface. Coating pigment particlesare easily dislodged from the coated sheet surface by rubbing and/orwhen the coated sheet is folded, slit, or die cut. Airvol® 203 is apartially hydrolyzed (87.0-89.0% hydrolysis) polyvinyl alcohol producedby Air Products and Chemicals, Inc. Allentown, Pa.

The results showed that the present processed zeolite pigment could actas an ink jet coating pigment.

Laboratory coating formulation experiments were performed to determinethe viscosity at highest obtainable coating solids. The present zeolitewas dispersed with 5% DMDAAC at 50.7% solids. A 30% solution of Airvol®203 PVOH¹⁵ was prepared by dispersing the granules in cold water,heating to 85° C. and holding at 85° C. for 30 minutes. The dispersedzeolite slurry and PVOH solutions were blended to obtain pigment tobinder ratios of 2:1, 4:1, 6:1 and 8:1 with no dilution water added.

Viscosity determination of the coating formulations was made using aBrookfield RVT viscometer with a #5 spindle. The data obtained fromthese experiments is contained in Table 3a and can be compared to datafrom three major suppliers of silica pigments as contained in Table 3band to data from Engelhard™ regarding a modified kaolin based pigment ascontained in Table 3c.

TABLE 3a Brookfield Viscosity of Zeolite Formulations Pigment: BinderRatio % Coating Solids 20 rpm 100 rpm 2:1 40.17 4000 1750 4:1 41.87 54001972 6:1 43.09 6760 2100 8:1 45.93 7880 2368

TABLE 3b Coating Formulation Solids Content from Silica Suppliers' DataSheet Supplier Grace-Davison J.M. Huber ICI Crosfield Product Sylojet ™Optisil ™ Gasil Pigment: Binder Ratio  2.49:1   1.00-1.67:1  2.5:1 %Solids 18.4 14-18 18

TABLE 3c Coating Solids Recommendation from Engelhard Data SheetSupplier Engelhard Product Digitex ™ Pigment: Binder Ratio 2.5:1 Coating% Solids 30 to 33%

As can be seen from these data, the present zeolite pigment providescoating color solids more than twice as high as any of the currentlyused silica pigments. It provides coating solids 21% higher than thehighest coating solids claimed from the Engelhard Digitex™ hybrid kaolinpigment. The present zeolite pigment provides shear-thinning rheology tofacilitate application by blade, rod, or metering size press coaters.The higher solids attainable with the zeolite pigment of the presentinvention provide substantial operating benefits to producers of ink jetpapers including less soak in and reduced roughening of the base sheetduring application resulting in a smoother coated sheet, improved coaterrunnability, decreased energy consumption in drying, higher operatingspeeds and higher production rates and capability to coat on high speedpaper machines rather than only on low speed off machine coating lineswhich reduces waste and costs.

Test printing of the drawdowns on Canon and Epson ink jet printersshowed that density improved as the pigment to binder ratio wasdecreased from 8:1 to 2:1. At 2:1 pigment to binder ratio, the presentzeolite pigment drawdowns came close to the value for commercial papersWeyerhaeuser Satin Ink Jet™ and International Paper Great White™ MPremium Matte Ink Jet Paper. The commercial papers had been produced onfull-scale machinery with optimized formulations and calendered toimprove performance. Due to these results, it was determined that pilotcoating trials should be performed.

The zeolite of the present invention was evaluated as a coating pigmentand filler with an emphasis on coating ink jet papers as a replacementfor silica. For the pilot coating, Cylindrical Laboratory Coater (CLC)trials were performed. The CLC¹⁶ is a laboratory device that simulatescoating at commercial machine speeds while consuming only small amountsof coating materials. It provides not only coated paper samples forevaluation, but also indications of runnability in commercialproduction. The coating experiments were performed using the zeolite ofthe present invention as the sole pigment with polyvinyl alcohol binderat varying pigment to binder ratios. More specifically, the experimentaldesign used was based on E-Chip using the following parameters: 1)pigment: binder ratios of 2:1, 5:1, and 8:1; 2) polyvinyl alcohol typesfrom Air Products™ including Airvol® 103 (fully hydrolyzed 98.0-98.8%hydrolysis)

¹⁶ The Cylindrical Laboratory Coater is manufactured by Sensor &Simulation Products, a division of Weyerhaeuser Co., Tacoma, Washington.and Airvol® 203 (partially hydrolyzed 87.0-89.0% hydrolysis); 3) Amp 95™and DMDAAC as dispersants; 4) coat weights of 6, 9 and 12 grams/ squaremeter; 5) 23 combinations of conditions; and 6) 29 total runs. The CLCtrials were run at 2500-3600 feet/minute. Blade metering was done with0.015-inch thick coating blade and a 0.018-inch thick backing bladeusing a 0.4-inch extension. The results of these trials are indicated inTable 4.

TABLE 4 Results of Cylindrical Laboratory Coater (CLC) Pilot TrialsPigment: binder ratio PVOH Speed Runnability/Coverage 2:1 203 2000Excellent 2:1 203 2500 Excellent 6:1 203 2000 Excellent 6:1 203 3000Good 6:1 203 3200 Good 6:1 203 3600 Uneven 8:1 203 2000 Excellent 8:1203 2800 Good 8:1 203 3200 Uneven at start 8:1 103 3200 Good 8:1 1033400 Uneven

The trials demonstrated that excellent runnability and coverage could beachieved at 2500 feet/minute, a speed substantially higher than the900-1500 feet/minute common on off-machines producing coated ink jetpapers. Optimization of the coating formulation of the present inventioncan increase the speeds at which the present zeolites can be used tocoat ink jet paper.

In evaluating the present zeolite for use in coating ink jet paper itwas important to take into account the effects of calendering¹⁷.Commercial coated ink jet papers are usually soft nip calendered toimprove image density. In order to determine the effects of calendering,test prints were made with both uncalendered and laboratory calenderedCLC coated papers. As expected, calendering improved print density. Thesamples were printed on three different commercial ink jet printers,Canon BJ500™, HP 932C™, and Epson 800™. Two commercial premium coatedink jet papers, Weyerhaeuser Satin Ink Jet™ and International PaperGreat White™ Premium Matte Ink Jet Paper, and a plain paper speciallysurface sized for ink jet printing were printed as bench marks. The inkdensities of the printed sample were compared using an X-Ritedensitometer. The ink densities of the present zeolite coated paperswere found to be statistically equal to or better than the premiumcommercial papers for all three printers. The best quality was achievedat 2:1 pigment to binder ratio. The results of this experiment arecontained in Table 5, which presented the data for thelaboratory-calendered samples. Laboratory calendering increaseddensities of all four colors on all three printers. No attempt was madeto optimize the zeolite formulations in contrast to the commercialsilica coated papers that are made with optimized formulations andmanufacturing procedures. In commercial practice, each papermanufacturer will optimize its formulation to match the characteristicsof the base paper to be coated and the coating equipment to be used.

¹⁷ Calendering is the process of compacting and smoothing paper duringmanufacture by passing it through a stack of polished metal rollerscalled calenders.

TABLE 5 Printability Tests of CLC Coated with Zeolite CLC SAMPLES(Flexible Blade Coated) Airvol 203 2:1 Pigment to Binder (35% solids)Coat Weight-gsm Cyan Magenta Yellow Black Printed on HP932C (600 × 600dpi) Average Reflective Densities-X-Rite Densitometer CLC Coated Samples3.6 1.334 1.398 0.962 1.536 4.6 1.312 1.412 0.974 1.522 5.4 1.316 1.3380.964 1.498 8.8 1.302 1.404 0.958 1.566 13.9 1.354 1.438 0.980 1.530Commercial Paper Control Samples Great White 1.110 1.162 0.896 1.494Weyerhaeuser 1.408 1.478 1.026 1.612 Plain Multi-Purpose 1.100 1.1500.900 1.500 Printed on EPSON 800 series (720 × 1440 dpi) AverageReflective Densities-X-Rite Densitometer CLC Coated Samples 3.6 0.9881.186 0.890 1.514 4.6 1.054 1.190 0.906 1.530 5.4 1.016 1.190 0.8981.548 8.8 1.042 1.184 0.892 1.498 13.9 1.048 1.200 0.898 1.522Commercial Paper Control Samples Great White 1.030 1.250 0.960 1.636Weyerhaeuser 0.888 1.020 0.860 1.280 Plain Multi-Purpose 0.946 1.0360.836 1.306 Printed on CANON BJC 5000 (720 × 1440 dpi) AverageReflective Densities-X-Rite Densitometer CLC Coated Samples 3.6 1.5241.568 0.934 1.420 4.6 1.532 1.422 0.898 1.410 5.4 1.474 1.510 0.9081.396 8.8 1.548 1.602 0.932 1.500 13.9 1.468 1.608 0.946 1.530Commercial Paper Control Samples Great White 1.438 1.486 0.972 1.560Weyerhaeuser 1.146 1.308 0.866 1.748 Plain Multi-Purpose 0.978 1.0520.802 1.448

In addition to its high quality performance, the zeolite pigmentprovides other significant compared to silica pigments. The zeolitepigment produces higher slurry solids with 50% for zeolite compared to30% maximum for silica and 42-45% for specialty hybrid kaolin pigmentswhich is a significant advantage in coating preparation. In addition,the zeolite pigment has higher coating solids with 36-40% for zeolitepigment compared to <20% for silica and 30-33% for specialty hybridkaolin pigments which means significant lower cost for drying and highercoating line operating speeds. Coating at higher solids not only savesenergy and increases production rate, but also results in a higherquality coated surface. The zeolite pigment also has a lower binderdemand. Coating prepared at pigment-to-binder ratios as high as 14:1 didnot show signs of cracking or flaking. With silica pigment, it isessential to use polyvinyl alcohol, which is the strongest availablebinder. An inexpensive starch cobinder can be used with the zeolitepigment of the invention. This capability can be a key to making ahigher fidelity mid-priced coated ink jet paper. The zeolite pigmentadditional has excellent rheology for use in various types of coatersincluding on-machine metering size presses. Silica coatings must beapplied on low speed (1000 to 1500 feet/minute) off machine coaters,which significantly increases costs. Coating with the zeolite pigment ofthe present invention on-machine at speeds in the 3000-4000 feet/minuterange combined with elimination of the extra costs associated with offmachine coating can facilitate serving a larger market.

The best ink jet densities were obtained using polyvinyl alcohol binderat 2:1 pigment to binder ratio. Density was reduced at higherpigment-to-binder ratios. This confirms the function of the superiorpigment void volume of the zeolite pigment. The implication of this isthat the zeolite pigment of the present invention can be effective inseveral applications including improvement of flexo ink vehiclereceptivity to prevent smudging in direct post print of corrugatedcontainers and use of the pigment as filler in newsprint and uncoatedground wood papers to eliminate print-through. Calcined kaolin, silicas,and silicates currently used in this second application are not costeffective.

Changes in retailing are driving the need for high quality multi-colorprinting on corrugated containers. In-line printing via flexography¹⁸without drying is the current preferred process. If the ink vehicle isnot rapidly absorbed the surface smudges. Use of coating pigments withgood void volume can prevent smudging. The best performing currentpigments are calcined kaolin and calcium carbonate; however, both are

¹⁸ Flexography is a method of printing on a web press using rubberplates with raised images. abrasive. Abrasive coating pigments make thesurface prone to metal marking producing gray streaks on the printedimage. Use of the zeolite pigment of the present invention which isnonabrasive as 10 to 15% of the total coating pigment should provide theneeded ink vehicle absorption without metal marking.

Coating drawdowns on linerboard were performed to determine the impactof the zeolite pigment on dynamic contact angle wetability, which is agood predictor of performance in direct print flexo on corrugated.Linerboard was precoated with 10 gsm of precoat formulation. Theprecoated samples were then top coated with 15 gsm of a standardformulation and also a formulation substituting 10 parts ZOBritepigment.

The coating formulations used were:

Precoating—Applied at 10 gsm

Dry Parts Component 100 Exsilon ™ chemically structured kaolin 15Acetate latex-Rohm & Haas 3103 3 Pro-Cote ® 4200 cold water dispersiblesoy protein 0.9 AZC crosslinker-HTI AZ-Cote ® 5800M 0.1 Polyacrylatedispersant-Dispex ® N-40 0.28 Ammonia-as required for pH 8.5

Top Coat Without Zeolite—Applied at 15 gsm

Dry Parts Component 40 No. 1 high brightness coating clay-Ultra-White 9040 Fine ground calcium carbonate-Hydrocarb ® 90 20 Titaniumdioxide-rutile-TiPure ® RPS Vantage 14 Acetate latex-Rohm & Haas 3103 4Pro-Cote 4200 cold water dispersible soy protein 0.7 Calcium stearatelubricant-Nopcote ® C-104-HS 1.6 AZC crosslinker-HTI AZ-Cote ® 5800M 0.1Polyacrylate dispersant-Dispex ® N-40 0.42 Ammonia-as required for pH8.5

Dry Parts Component 10 Zeolite pigment 35 No. 1 high brightness coatingclay - Ultra-White 90 35 Fine ground calcium carbonate - Hydrocarb ® 9020 Titanium dioxide - rutile - TiPure ® RPS Vantage 14 Acetate latex -Rohm & Haas 3103 4 Pro-Cote 4200 cold water dispersible soy protein 0.7Calcium stearate lubricant - Nopcote ® C-104-HS 1.6 AZC crosslinker -HTI AZ-Cote ® 5800 M 0.1 Polyacrylate dispersant - Dispex ® N-40 0.42Ammonia - as required for pH 8.5

The dynamic contact angle of the coated samples was measured and theresults shown in Table 6 and FIG. 1. It was found that substitution of10 parts zeolite in the top coat formulation provided a significantimprovement in dynamic contact angle wetability. This shows the zeoliteprovides the capability to capture flexo ink in direct print (withoutdrying) on a flexo-folder-gluer or case-making machine. The top coatcoated with the zeolite composition was evaluated for metal marking byrubbing the surface with a nickel coin. No metal marking was observed.

TABLE 6 Dynamic Contact Angle Measurements of Coated Linerboard Time NoTime 10 Parts Seconds Zeolite Seconds Zeolite 0.0 63.42 0.0 51.91 22.562.52 6.3 51.53 45.0 60.69 54.5 51.07 57.0 59.27 67.1 49.85 64.5 58.9879.8 50.51

Further trials were run on the CLC to determine the effect ofsubstitution of the present zeolite for No. 1 high brightness clay in astandardized paperboard topcoat formulation:

standardized Topcoat Formulation

Dry Parts Component 40 No. 1 high brightness coating clay - Ultra-White90 ® 40 Fine ground calcium carbonate - Hydrocarb ® 90 20 Titaniumdioxide - rutile - TiPure ® RPS Vantage 14 Acetate latex - Rohm & Haas3103 4 Pro-Cote 4200 cold water dispersible soy protein 0.7 Calciumstearate lubricant - Nopcote ® C-104-HS 1.6 AZC crosslinker - HTIAz-Cote ® 5800 M 0.1 Polyacrylate dispersant - Dispex ® N-40 0.42Ammonia - as required for pH 8.5

The control topcoat was made up at 55% solids and pH 8.5. Brookfieldviscosity was 600 cPs using a No. 6 spindle at 100 rpm. Experimentalcoatings were made by substituting 5, 10, 15 and 20 parts zeolitepigment for No. 1 coating clay. These coatings were also prepared at 55%solids and pH 8.5. Each of the coatings was evaluated on a Hercules highshear rheometer using Bob E, 6600 rpm and spring set 200. Rheogramsshowed all coatings to be shear stable. Torque at 6600 maximum rpm foreach of the coatings was:

Parts Zeolite Torque - kilodyne-cm 0 1750 5 1800 10 2128 15 2053 20 2507

The topcoats were applied to precoated recycled paperboard using a CLClaboratory coater with a blade application and a target coat weight of 4to 5 pounds per 1000 square feet. Three replicates were done for thecontrol and each of the four experimental coatings for a total of 15samples. Each sample was then calendered on a hard/soft nip calender at600 pli for three passes before evaluation.

The coated and calendered unprinted paperboard samples were tested fordynamic contact angle. The results of the dynamic contact angle showedthat the 15 parts of zeolite had the best absorption followed closely bythe 5 and 10 parts of zeolite pigment. The more rapid drop of thecontact angle with the specimens containing zeolite pigment shows thatthe zeolite pigment adds a greater absorption rate into the coatedsurface. Increasing the zeolite pigment fraction to 20 parts did notprovide better absorption than achieved with 15 parts zeolite pigment.

The coated and calendered unprinted paperboard samples were tested forbrightness with the following results:

Parts Zeolite Brightness 0 82.1 5 83.8 10 84.1 15 80.5 20 80.5

There was a gain in brightness from the control (0 parts zeolitepigment) with 5 and 10 parts of zeolite pigment, then the brightnessdropped with higher levels of zeolite pigment. This is encouraging fortwo reasons: (1) there is an increase in brightness with the addition ofsmall amounts of zeolite pigment and (2) this increase in brightnesscould allow for more intense calendering of the formulations with 5 and10 parts zeolite pigment to increase gloss.

The coated and calendered paperboard samples were printed on a GMS FlexoPrint Proofer. Ink density was measured with an X-Rite densitometer. Inkdensity for all samples was in the range of 2.2-2.3; a density change of2.0 points is considered significant. There is no apparent change in inkdensity with increasing amounts of zeolite pigment substitution. This isimportant in that the coated surface with the addition of zeolitepigment allows for increased absorptivity of the ink vehicle withoutabsorbing the ink pigment into the sheet. These results are also anindication that inclusion of the zeolite pigment would be useful inimproving water-based gravure printing quality.

Due to increasing postage and handling costs, the basis weight ofnewsprint and other uncoated groundwood printing papers continues to bereduced. At the same time, newspapers are doing more color processprinting. The thinner sheets are unfortunately prone to print-through.Use of a porous filler pigment cannot only help to reduce print-through,but can also increase opacity. Newsprint is made at acid pH whichprevents the use of calcium carbonate for this application since itprovides too alkaline of an environment. Calcined clay works inpreventing print-through, but it is difficult to retain and is alsoabrasive. The current products of choice are lower grade silicas andprecipitated silicates, but the use of the products is not costeffective. The zeolite pigment of the present invention is not onlynonabrasive, but also cost-effective.

Pilot paper machine trials were run comparing the use of the zeolite ofthe present invention to precipitated calcium carbonate (PCC) as filler.The trials showed significant advantages of the present zeolite pigmentas filler. These pilot machine filler trials were run without use ofretention aid polymers. It was found that the filler retention for thepresent zeolite was 2.5 to 4 times as high as PCC which facilitatesrunning a cleaner wet end with improved sheet formation and uniformoptical properties. The significantly higher retention achieved with thezeolite of the present invention is an indication that it can performwell as a substitute for silica in microparticulate retention systems.Silicas currently used in this application are not cost effective. Theimproved retention of the zeolite pigment is an indication that it wouldbe useful as an alternative to costly silica as a deinking aid.

In addition, porosity tests showed that the present zeolite produced amore open sheet, which would facilitate the use of this pigment inspecialty gas filtration papers and anti-tarnish papers. It was alsofound that the zeolite pigment of the present invention produced papersthat had higher tensile strength and tensile energy absorption orstretch. Papers filled with the present zeolite also had a highercoefficient of friction, which decreases the likelihood of misfeed andjams in copiers and also improves performance in converting equipmentand print shops. The zeolite of the present invention can also be usefulas a frictionizer for coefficient of friction control in recycledlinerboard.

The capability of the zeolite pigment to reduce print-through wasevaluated by printing samples from the pilot paper machine trials on aproof press and visually inspecting them for evidence of printshow-through. The control sample with no filler showed severeprint-through. The sample filled with 100 pounds of zeolite pigment(4.59% measured ash content) showed no evidence of print-through.Samples filled with PCC at levels up to 250 pounds per ton showed littleimprovement over the unfilled control with regard to print-through. Thesuperior performance of the zeolite pigment in minimizing print-throughis an indication that it would be useful in production of ultralightweight-coated publication papers.

A short pigmented size press coating trial was performed during thepilot paper machine run. The zeolite of the present invention wasformulated in a 2:1 ratio with size press starch and applied viaconventional pond size press. Runnability was good and the sheet wasfree from dusting. Samples of the pigmented size press coated paper wereprinted on the three ink jet printers. This preliminary trial workshowed that the zeolite of the present invention can be used as pigmentfor size press coating.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitedsense. Various modifications of the disclosed embodiments, as well asalternative embodiments of the inventions will become apparent topersons skilled in the art upon the reference to the description of theinvention. It is, therefore, contemplated that the appended claims willcover such modifications that fall within the scope of the invention.

We claim:
 1. Paper comprising a zeolite selected from the familyHeulandite and having a BET surface area in the range of about 40-50m²/g.
 2. The paper of claim 1 wherein said zeolite is present in anamount up to and including 20 parts per hundred.
 3. The paper of claim 1wherein said zeolite is present as a coating on said paper.
 4. The paperof claim 1 wherein said zeolite is present as a filler within saidpaper.
 5. The paper of claim 1 wherein said zeolite is present as asupplementary pigment to said paper.
 6. The paper of claim 3 wherein acoating solids percentage of at least 36% is achieved.
 7. The paper ofclaim 1 wherein said zeolite is clinoptilolite.
 8. The paper of claim 7further comprising at least one binder and a polymeric dispersing agent.9. The paper of claim 8 wherein said binder is selected from the groupconsisting of acetate latex, styrene-butadiene, acetate-acrylate, vinylacrylic, acrylic latexes, and polyvinyl alcohol.
 10. The paper of claim9 wherein said polymeric dispersing agent is selected from the groupconsisting of polyacrylate dispersant, poly-dimethyl-diallyl ammoniumchloride, and 2-amino, 2-methyl, 1-propanol.
 11. The paper of claim 10further comprising a cobinder, wherein said cobinder is selected fromthe group consisting of polyvinyl pyrolidone and starch.
 12. The paperof claim 11 further comprising an optical brightener.
 13. The paper ofclaim 12 further comprising an insolubilizer; a lubricant; and analkali.
 14. The paper of claim 13 wherein said lubricant is selectedfrom the group consisting of calcium stearate, polyethylene dispersions,lecithin oleate, aliphatic esters, and polyglycerides.
 15. The paper ofclaim 14 wherein said insolubilizer is selected from the groupconsisting of ammonium zirconium carbonate, glyoxal, glyoxalated resins,melamine formaldehyde, and urea formaldehyde.
 16. The paper of claim 15wherein said polymeric dispersing agent is selected from the groupconsisting of polyacrylate dispersant, poly-dimethyl-diallyl ammoniumchloride, and 2-amino, 2-methyl, 1-propanol.
 17. The paper of claim 16wherein said alkali is selected from the group consisting of ammonia andsodium hydroxide.